Windmill and wind turbine systems are generally either horizontal axis systems where the rotating shaft is arranged in a horizontal direction, or vertical axis where the rotating shaft is arranged in a vertical direction.
Vertical axis wind turbines rotate in the same position irrespective of wind direction whereas horizontal axis wind turbines must be positioned to face the wind direction in order to rotate. Vertical axis wind turbines also rotate with relatively silent movement and fit into most landscapes. For these reasons, vertical axis wind turbines are generally considered better suited for urban use when compared to horizontal axis wind turbines.
Vertical axis wind turbines are generally available in two varieties, drag or lift. The drag variety utilizes drag created by the blades for rotation. To this end the blades can be arranged as paddles or as a Savonius design. The lift variety utilizes lift created by the blades for rotation. Blade shape arrangements for lift variety wind turbines are apparent in a Darrius (also spelled Darrieus) or gyromill design, and often resemble the shape of aircraft airfoils.
For vertical axis drag type wind turbines, when the speed of a tip of a blade reaches wind speed, i.e. when tip speed ratio equals 1 (TSR=1), no more force can be applied to the blade. Thus, wind turbine rotation is limited since the blade speed is limited to wind speed thereby limiting the electricity generation capabilities of the wind turbine.
For vertical axis lift type wind turbines, however, the speed of a tip of a blade can exceed wind speed. Specifically, TSR can exceed and is preferably greater than 1. At these speeds, the wind turbine can generate a high level of electricity, but the aerodynamics of the blades are usually inefficient when blade speed is below wind speed, i.e. when TSR<1. Thus, a vertical axis lift wind turbine only generates substantial amounts of electricity when TSR>1. Moreover, in low wind speed environments, blade aerodynamics are inefficient and cannot produce enough torque to initiate rotation, and the wind turbine usually requires a separate power source to initiate rotation.
To compensate for the deficiencies of vertical axis lift wind turbines, additional components are generally added to facilitate initial rotation. For example, components like that of the Savonius design may be attached on the same axis of the vertical axis wind turbine to increase startup torque and improve startup efficiency. However, adding components like that of the Savonius design increases the number of parts, raises production costs and increases design complexity. Moreover, in high wind speed environments, the added components cause drag and resistance to the rotation of the wind turbine, lowering the efficiency of the wind turbine.
Another method of improving vertical wind turbine designs is to include concaved curves behind the blades similar to that of a Savonius design. The supporting point of the blade in relation to the vertical shaft is shifted 10-30% to the front or the back from the center point, and the angle of the supporting arm of the blades to the shaft is kept constant by utilizing springs. Although this arrangement creates enough start up torque by generating drag in low wind speeds, in high wind speeds the same concaved curves produce vortexes that lower wind turbine efficiency. Additionally, especially if the wind turbine is susceptible to rain or moisture, the springs necessary for the system to support the angle of the blade in relation to the shaft can deteriorate and break.
An additional improvement to wind turbine designs, specifically to improving rotational efficiency, includes new blade shape designs. Specifically, the blade surface that faces the shaft remains separated at the back edge. Since it has no concaved curve like that of a Savonius design, there still exists the issue of inadequate torque to initiate rotation prior to TSR reaching 1.
As discussed above, in a drag type vertical axis wind turbine, wind turbine rotation is limited to TSR=1, limiting electricity generation. When TSR>1, lift type vertical axis wind turbines are efficient, but when the TSR<1, the aerodynamics are inefficient and the aerodynamics are inadequate to produce enough startup torque to initiate wind turbine rotation, necessitating a secondary power source to begin rotation.
Accordingly, there is a need for a wind turbine to solve the issues that have been described above that will eliminate inadequate torque at startup with the lift type vertical axis wind turbine while maintaining efficient electricity generation at high rotation speeds, and allow TSR to exceed 1. Stated alternatively, what is needed is a wind turbine that operates efficiently throughout TSR<1 to TSR>1 to accommodate varying wind speed conditions.